The so-called biological sewage treatment systems, in which dissolved organic substances are removed from the liquid or sewage are well known. In these systems the sewage is led into an aerating basin that is saturated with a mass of activated sludge with bacteria; the aerated sewage is led into a secondary sedimentation basin where the sludge is separated by sedimentation and recirculated into the aerating basin while the purified water is discharged from the system. An apparatus and a method of this kind are disclosed in Hungarian Pat. No. 166,160. These serve to circulate and aerate liquids in the course of their operation in biological sewage treatment plants i.e. oxidating trenches. In this known installation the streaming energy of the liquid to be treated is increased by means of a pump. In addition, a water jet pump is actuated by the energy of moving liquid, while air is introduced through the pump-wheel. In the course of the process the aeration step and circulation step are not separated. German Pat. No. 24 18 586 discloses apparatus in which biological sewage treatment can be carried out e.g. in a pool, wherein the water is divided into pathways by means of separation walls. In the path, in a predetermined mutual distance from each other, air-water injecting means produce a horizontal stream i.e. the water is allowed to stream in a horizontal direction. In this case aeration and circulation are not separated.
German Patent DE-OS 28 16 390 discloses a sewage treatment plant using the activated sludge process, in which several aerating trenches running parallel and connected in series are inter-connected by means of sections of circular arches (90.degree. or 180.degree.). At the ends of the separation walls separating the single aerating trenches, aerating wheels with a vertical axle are arranged, while in every second trench are pipe propellers with a horizontal axle. Accordingly, in this process circulation of the liquid and aeration are separated. The system is known as the Carrousel system.
The known biological sewage treatment plants using activated sludge do not solve optimally the task of aeration and sedimentation. Efficiency is low regarding both energy utilization and purification. Simultaneously space requirements are large and several drawbacks are involved in respect to operational techniques.